CARACAS, Venezuela — Secretary of State John Kerry has markedly stepped up his criticism of Venezuela over its response to more than two weeks of antigovernment demonstrations, as the protests continued to grow this weekend.

“I am watching with increasing concern the situation in Venezuela,” Mr. Kerry said in a statement on Friday night. “The government’s use of force and judicial intimidation against citizens and political figures, who are exercising a legitimate right to protest, is unacceptable and will only increase the likelihood of violence.”

Last week the government arrested a prominent opposition leader, Leopoldo López, whom it charged with inciting protesters to violence. Mr. Kerry did not name Mr. López in his statement, but he called on the government to release jailed opposition members.

On Saturday, thousands of people in Caracas, the capital, attended one of the largest opposition rallies yet, a sign that the protests, which began this month with isolated student demonstrations against high crime, may continue to gain strength.

The protesters filled many blocks of one of Caracas’s main avenues, many of them wearing white shirts to symbolize peace. The protests are expressing the widespread discontent with the government of President Nicolás Maduro, a socialist who has vowed to carry on the revolution of the country’s charismatic longtime president, Hugo Chávez, who died last year.

“The one who is responsible for all this is Maduro, for his incendiary discourse,” said Derek Redman, 77, who attended the march. His son, Roberto Redman, 31, was shot to death during a protest on Feb. 12. “I will not stop marching now that my son is dead. This situation can change if we don’t lower the pressure.”

Demonstrations were held in several other cities as well.

The government continues to ignore the vast majority of demonstrators, who have acted peacefully, and instead focuses attention on acts of violence and property damage that have been associated with some of the protests. At least four people have been shot to death.

In a speech in Caracas after a march organized by the government that was billed as a rally of women for peace, Mr. Maduro held up a small explosive filled with nails that he said was seized by security forces and could have been used by opponents of his government to attack soldiers.

“This is terrorism,” he said. “Fascist, spoiled, rich kids!”

María Eugenia Díaz contributed reporting.

A version of this article appears in print on , on Page A8 of the New York edition with the headline: Kerry Says Venezuela Crackdown ‘Unacceptable’. Order Reprints | Today’s Paper | Subscribe